The present invention relates to a magnet structure for producing a high strength, high homogeneity magnetic field. More particularly, the present invention is related to the design and construction of electromagnets useful for whole-body, nuclear magnetic resonance (NMR) imaging systems, especially such systems for medical diagnostic applications.
In the generation of tomographic and planar images and other data from NMR imaging devised, it is necessary to have a high strength and high uniformity magnetic field. In NMR systems for whole-body medical diagnostic imaging, the magnetic field strengths typically range from about 0.04 to about 1.5 Tesla, or more. In order to reduce geometric distortion and to prevent other undesirable artifacts in the image or data, and also to limit the power required in the gradient and radio frequency coils, it is also necessary that the main magnetic field exhibit a high degree of field strength uniformity. The main magnetic field B.sub.o for NMR imaging is typically produced by a set of one or more principal coils disposed on a cylindrical surface. These coils may be either resistive or superconductive. Additionally, permanent magnets may also be employed to generate the principal magnetic field component B.sub.o. It is further convenient that this magnetic field be oriented in an axial direction with respect to the cylinder on which the coils are disposed. Ideally, this condition should hold throughout the cylindrical volume.
However, whether resistive, superconductive or permanent magnets are employed, it is necessary to build the magnet to a carefully specified configuration and to strive to minimize deviations from the specified shape due to manufacturing variability. However, even when extraordinary steps are taken to ensure proper construction and magnetic field uniformity, a residual field error remains. Accordingly, it is conventional practice to employ relatively low power correction coils to perturb the field in a manner which increases the overall field uniformity.
Because field errors arise from uncontrollable deviations of the magnet or from unpredictable variablility in the magnetic boundary, the size and shape of the perturbation required of the correction coils cannot be predicted before magnet construction. Accordingly, correction coils are generally employed to produce a field which is adjustable both in magnitude and shape so as to achieve the formation of a correction field leading to a more uniform overall (net) field.
In describing the field within the cylindrical volume of interest, experimenters in the field of NMR imaging have almost universally employed a spherical coordinate system together with a series of spherical harmonic functions indexed by two parameters to describe the field within the cylinder. More particularly, it is known to employ correction coils which are specifically chosen so that each correction coil circuit produces a field with a shape corresponding approximately to one of the terms in the set of spherical harmonic functions. Since the set of spherical harmonic functions is orthogonal over the region of interest, correction coil design is therefore greatly simplified. In particular, since the functions are orthogonal each correction coil circuit may be designed to correct for a single term in the expansion describing the error field. Therefore, such correction coils have the desirable property that adjustment of one correction coil circuit is nearly independent of the adjustment of any other circuit. However, the disadvantage of such correction coils is that the configuration of coils required to achieve this end can be more complicated than is necessary to achieve a specified minimum error condition. In the spherical harmonic expansions which are employed for correction coil design, the number of physically distinct coils in a circuit increases very rapidly with the order of the highest harmonic term for which correction is desired. The reason for this rapid increase in the number of coils is that the spacial structure for the higher order spherical harmonic functions is substantially more complicated.
However, it is the purpose of this invention to achieve the capability of adjusting many parameters of the field shape with a set of correction coils the structure of which is much simpler than a set of coils designed to act separately on each spherical harmonic function to be corrected.